Remote cool down of a purified directionally solidified material from an open bottom cold crucible induction furnace

ABSTRACT

Solid or semi-solid feedstock is melted in an open bottom electric induction cold crucible furnace. Directionally solidified multi-crystalline solid purified material continuously exits the bottom of the furnace and may optionally pass through a thermal conditioning chamber before being gravity fed into a transport mold where an ingot of the purified multi-crystalline solid material is transported to a remote holding area after the transport mold is filled with the multi-crystalline material and cut from the continuous supply of material. Cool down of the ingot is accomplished remote from the open bottom of the cold crucible furnace.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/222,994, filed Jul. 3, 2009, hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to directional solidification of a multi-crystalline ingot of a purified material from an open bottom cold crucible induction furnace where the cool-down of the hot solid ingot is accomplished remote from the furnace.

BACKGROUND OF THE INVENTION

Electromagnetic casting of a multi-crystalline material such as silicon can be accomplished in an open bottom electric induction cold crucible furnace. Feedstock is continuously supplied to the crucible's interior volume and melted in the cold crucible while a solidified hot mass of the multi-crystalline material exits from the open bottom of the crucible. The solidified hot mass of the multi-crystalline material is typically cooled down as an integral step in the casting process after it exits from the electric induction cold crucible without immediate removal from the casting process line.

One object of the present invention is to produce a purified multi-crystalline solid ingot from feedstock material where the feedstock is melted and purified in an open bottom cold crucible induction furnace and a hot multi-crystalline solid mass exits from the bottom of the furnace for deposit in a mold that is removed from the casting process line for remote passive cool-down and directional solidification of the hot multi-crystalline solid mass in the mold without further application of external heating or cooling of the mold.

BRIEF SUMMARY OF THE INVENTION

In one aspect the present invention is apparatus for, and method of, purifying a crystalline material by directional solidification. Solid or semi-solid feedstock is melted in an open bottom cold crucible electric induction furnace. Hot purified multi-crystalline solidified material continuously exits from the open bottom of the furnace, and may optionally pass through a thermal conditioning chamber, before being deposited in a transport mold and moved to a remote holding area for passive cool-down and directional solidification of the hot purified multi-crystalline solidified material.

The above and other aspects of the invention are set forth in this specification and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing brief summary, as well as the following detailed description of the invention, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings exemplary forms of the invention that are presently preferred; however, the invention is not limited to the specific arrangements and instrumentalities disclosed in the following appended drawings:

FIG. 1 illustrates on example of an apparatus used in the present invention.

FIG. 2 illustrates another example of an apparatus used in the present invention.

FIG. 3 illustrates another example of an apparatus used in the present invention.

FIG. 4 graphically illustrates the change in thermal coefficient of expansion of solid silicon relative to the temperature of the silicon.

DETAILED DESCRIPTION OF THE INVENTION

There is shown in FIG. 1 one example of the apparatus and process of the present invention. Feedstock 90 is supplied in a suitable form from a suitable source to electric induction cold crucible 12, which is surrounded by one or more induction coils 14 that are connected to one or more suitable alternating current sources 16. The open bottom electric induction cold crucible 12 is similar in construction and operation to a closed bottom electric induction cold crucible furnace, for example, as described in U.S. Patent Application Publication No. 2005/0175064 A1 except that there is no bottom structure to inhibit bottom exit of a continuous hot solidified mass of a purified multi-crystalline material from the bottom of the cold crucible.

Initially feedstock 90 may be in a semi-solid state (particularly if the feedstock material is non-electrically conductive in the solid state, and electrically conductive in the liquid state, such as silicon feedstock) to establish a generally steady state solidification front 90′ of molten feedstock 90 a (horizontally line shaded region) over purified multi-crystalline hot solid mass 90 b (stipple-shaded region) exiting from the bottom of the cold crucible furnace. The multi-crystalline hot solid mass will gravity free-fall into mold 81. The mold can have a moveable inner bottom 81 a with powered driver 92 controlling the drop speed of the moveable inner bottom, and therefore controlling the rate of vertical formation of the multi-crystalline solid mass in the mold. The bottom opening of the induction cold crucible 12 may be of any shape, for example, to produce a cylindrical or rectangular solid mass.

In-process fill mold 81; pre-filled mold 81″ and post-filled mold 81′ can each be formed from a suitable liner material 81 b as consistent with the type of material mass 90 b contained within the mold. Thermal insulation 81 c may surround at least the exterior wall of the mold. In some examples of the invention, bottom 81 a may be formed from a susceptor. The susceptor may optionally be suitably heated, for example either resistively or inductively, prior to filing of the mold.

When the multi-crystalline solid mass has achieved a desired vertical height in mold 81, cut-off saw 18 is activated to slice through a horizontal plane of the multi-crystalline solid mass. Downward movement of the multi-crystalline solid mass can be controlled at a sufficiently slow rate relative to the cut through speed of the cut-off saw so that a substantially horizontal cut of the multi-crystalline solid mass is achieved with the cut-off saw.

Mold 81, when filled with a hot multi-crystalline solid mass, is moved away from the furnace, as represented by filled mold 81′ in FIG. 1 and covered with thermal insulating lid 81 d so that the multi-crystalline solid mass in mold 81 cools down and directionally solidifies passively without application of controlled zone external heating or cooling. A multi-crystalline metal or metalloid, such as silicon, will initially solidify (amorphous-to-crystalline phase transition) at a high temperature (nominally 1,380° C. for silicon), which is the hot multi-crystalline solid mass deposited in the mold prior to cool-down and directional solidification. Most important in any material directional solidification process is to avoid an effect know as “shrink defects” caused by volume contraction of the material as it transitions inwardly from the liquid state to the hot solid state by controlling the volume rate of transition from liquid to hot solid state, and then controlling the cool-down (directional solidification) rate of the hot solid mass of material in the mold. For example the graph in FIG. 4 illustrates the expansion and contraction characteristics of solid silicon over a temperature range. Thermal monitoring systems can be provided to monitor the passive rate of cool-down of the hot material mass in the mold, so that, for example, the temperature of the hot solidified mass of purified multi-crystalline material exiting the open bottom of the crucible can be adjusted for an optimum passive cool-down rate. For example thermocouples or other temperature sensors 83 can be buried along the height of the mold's wall to monitor the rate of cool-down in the mold. The entire process may be automated to minimize manpower required for operation of the process as long as a constant material feedstock is available.

The molds can be supplied to the furnace (such as pre-filled mold 81″) and removed from the furnace on a suitable conveyance apparatus. The conveyance apparatus can move filled molds to a remote cool-down storage zone that automatically moves molds containing material that have completed the cool-down process to a process finishing zone, for example, for further trimming of the ingot for shipping and recycling of ingot trimmings of sufficient purity as feedstock 90. The entire process may be automated to minimize manpower requirements for the operation.

In some examples of the invention, the filled mold may be inserted into a thermally controlled chamber, or tunnel, at the remote area that further regulates the rate of heat dissipation from the multi-crystalline mass in the mold. The chamber or tunnel can provide passive thermal control, for example, by lining the chamber or tunnel with thermal insulating material.

Generally the mold filing and remote cool-down process is not an extremely fast process.

For example for a mold with a cross section of 32 centimeters square, and an interior height of 1 meter, process time to fill the mold with a multi-crystalline solid silicon mass can be on the order of six hours. Completion of a remote passively controlled cool-down of the same mass of silicon in a mold can be on the order of thirty-six hours.

In some examples of the present invention, thermal conditioning of the multi-crystalline solid mass exiting from the cold crucible furnace may be required. This may be achieved, for example, as shown in FIG. 2 wherein graduated vertical heat zone control apparatus 20 is provided. In this example the zone heat source is an induced electromagnetic field selectively applied to vertical zones of susceptor 21 as the multi-crystalline solid mass drops through apparatus 20. The electromagnetic field can be established via current flow through induction coils 22 a, 22 b and 22 c (separated from the susceptor by thermal insulator 23) with current supplied from alternating current source 24 and controlled by switching devices 26 a, 26 b and 26 c. Generally, applied induced heat to the susceptor is reduced in the direction from top to bottom of apparatus 20 to establish a temperature gradient along the height of the susceptor wall. Susceptor 21 performs a secondary function of retaining the outer wall shape of the solidified mass of multi-crystalline material as it passes through apparatus 20.

In a directional solidification process, impurities generally migrate to the top of the liquid material phase of the melt. One method of removing these impurities in the present invention is illustrated in FIG. 3. In this example at least the open top of induction cold crucible 12 is contained within sealed pressure chamber 30 so that a positive (above atmospheric) pressure can be applied to the surface of melt 90 a below which surfaces the impurities tend to migrate. For example valve 32 may be used to supply an inert gas to the pressure chamber, which forces impurities-enriched melt 90 a up tube 34 and out of the pressure chamber. In the particularly arrangement shown in FIG. 3 tube 34 is arranged as a siphon tube. Consequently once the pressurized gas primes siphon tube 34 the interior of chamber 30 can be vented to atmospheric pressure via connecting open valve 32 to atmosphere and the siphon drain of impurities-enriched melt 90 a will stop when the height of melt 90 a in the cold crucible reaches y₁, which is the vertical position of the opening of siphon tube 34 in the uppermost portion of the melt containing the impurities.

Tube 34 may be formed at least in part along its length from a high temperature electrically conductive material. If the tube is formed from an electrically conductive material, the tube may be electrically heated by connecting the external end of the tube to a first terminal of electric power source 36 that has a second terminal connected to system ground. If the cold crucible induction furnace is system grounded, for example by system ground connection 12 a to the wall of the crucible, the electrical conducting path through the furnace skull (solid material formed during cold crucible operation) and melt in the crucible that is in contact with the end of the tube in the melt will establish a Joules heating current flow from power source 36 (via the system grounded return) through the electrically conductive tube that will keep the impurities-enriched melt flowing through the tube without solidification (freezing). One must make certain that the system ground is safely installed and operating correctly in any particular installation as established by applicable standards. Impurities-enriched melt drawn from the crucible may be sent to an impurities separator so that sufficiently pure feedstock can be separated from the impurities and fed back into the feedstock supply to the cold crucible.

While the present invention is particularly applicable to a cool-down process for directionally solidified silicon, the invention can also be applied to other metalloids or metals, and alloys thereof, capable of purification by directional solidification that require a controlled cool-down process.

The present invention has been described in terms of preferred examples and embodiments. Equivalents, alternatives and modifications, aside from those expressly stated, are possible and within the scope of the invention. 

1. A solidification process for a multi-crystalline material from a melt produced in an open bottom induction cold crucible, the solidification process comprising the steps of: supplying a feedstock at the top of the open bottom induction cold crucible; forming the melt in the open bottom induction cold crucible by magnetic flux coupling with the feedstock and melt combination when an alternating current flows through one or more induction coils surround the exterior of the open bottom induction cold crucible; forming from the melt a continuous supply of a hot solidified mass of purified multi-crystalline material at the open bottom of the open bottom induction cold crucible; gravity feeding the hot solidified mass of purified multi-crystalline material into a mold; severing the hot solidified mass of purified multi-crystalline material from the continuous supply of the hot solidified mass of purified multi-crystalline material at the open bottom of the open bottom induction cold crucible; and moving the mold for remote directional solidification of the hot solidified mass of purified multi-crystalline material in the mold without application of an external heating or cooling source.
 2. The solidification process of claim 1 wherein the mold comprises a moveable inner bottom, the moveable inner bottom in communication with a powered drive so that the powered drive controls the rate of the gravity feeding of the hot solidified mass of purified multi-crystalline material in the mold.
 3. The solidification process of claim 1 wherein the step of moving the mold for remote directional solidification of the hot solidified mass of purified multi-crystalline material in the mold without application of an external heat source further comprises inserting the mold into a passive thermally controlled chamber to regulate the rate of heat dissipation from the hot solidified mass of purified multi-crystalline material in the mold.
 4. The solidification process of claim 1 further comprising thermally conditioning the continuous supply of the hot solidified mass of purified multi-crystalline material at the open bottom of the open bottom induction cold crucible prior to the step of gravity feeding the hot solidified mass of purified multi-crystalline material into the mold.
 5. The solidification process of claim 1 further comprising the step of siphoning impurities in the melt by applying a positive gas pressure to the surface of the melt at the top of the open bottom induction cold crucible to establish the siphon flow from the melt.
 6. The solidification process of claim 5 wherein the step of siphoning impurities in the melt further comprises electrically heating a siphon tube having a first end submerged in the melt and a second end opposing the first end disposed external to the interior volume of the open bottom induction cold crucible by connecting an electrical ungrounded potential terminal of a power supply to the second end of the siphon tube and an electrical ground potential terminal of the power supply to a system electrical ground, the open bottom induction cold crucible connected to the system electrical ground, whereby an electrical circuit is established across the length of the siphon tube by contact of the first end of the siphon tube with the melt and the melt with the furnace skull formed in the induction cold crucible heating process.
 7. A system for producing an ingot of solidified multi-crystalline material, the system comprising: an open bottom induction cold crucible furnace; a feedstock supply apparatus for delivering a feedstock to a melt in the open bottom induction cold crucible furnace; at least one induction coil surrounding the exterior height of the open bottom induction cold crucible furnace to form the melt in the open bottom induction cold crucible furnace by magnetic coupling with the feedstock and melt combination a flux field established by alternating current flow in the at least one induction coil; a cut-off saw for severing a hot solidified mass of purified multi-crystalline material exiting from the open bottom of the open bottom induction cold crucible furnace; and a removable mold for deposit of the severed hot solidified mass of purified multi-crystalline material from the open bottom of the open bottom induction cold crucible furnace and transporting the severed hot solidified mass of purified multi-crystalline material to a location remote from the open bottom of the open bottom induction cold crucible furnace for passive directional solidification of the severed hot solidified mass of purified multi-crystalline material.
 8. The system of claim 7 wherein the removable mold further comprises a thermal insulation surrounding at least the exterior walls of the mold.
 9. The system of claim 7 wherein the removable mold further comprises a moveable inner bottom connected to a powered driver for controlling the rate of insertion of the hot solidified mass of purified multi-crystalline material in the mold.
 10. The system of claim 7 further comprising a passive thermally controlled chamber for insertion of the mold.
 11. The system of claim 7 further comprising: a susceptor thermal conditioning vessel disposed between the open bottom of the open bottom induction cold crucible furnace and the removable mold for passage of the hot solidified mass of purified multi-crystalline material from the open bottom of the open bottom induction cold crucible furnace; one or more induction coils surrounding the exterior of the susceptor thermal conditioning vessel to establish a thermal gradient along the height of the vessel by inductively coupling magnetic flux generated by alternating current flow through the one or more induction coils so that the hot solidified mass of purified multi-crystalline material is thermally conditioned prior to deposit in the moveable mold.
 12. The system of claim 7 further comprising: a sealed enclosure over the open top of the open bottom induction cold crucible furnace; a siphon tube having a first and second opposing ends, the first opposing end of the siphon tube positioned in the interior volume of the open bottom induction cold crucible furnace at a height for siphoning impurities from the melt, the second opposing end of the siphon tube positioned external to the sealed enclosure; and a means for introducing a gas into the sealed enclosure to raise the level of melt in the open bottom induction cold crucible furnace by applying a positive gas pressure to the surface of the melt to a height sufficient to establish a siphon flow of the melt from the open bottom induction cold crucible furnace with the siphon flow terminating after the positive gas pressure is removed and the height of the melt drops to the height of the first opposing end of the siphon tube.
 13. The system of claim 12 further comprising: a power supply having an ungrounded electrical output terminal and an electrically grounded electrical output terminal; the ungrounded electrical output terminal connected to the second opposing end of the siphon tube; and the electrically grounded electrical output terminal connected to a system ground connection, the open bottom induction crucible connected to the system ground connection, whereby an electric current flows from the power supply through the siphon tube when the melt makes contact with the siphon tube via the furnace skull formed around the interior wall of the open bottom induction crucible during operation of the open bottom induction cold crucible furnace. 